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   难度：Medium
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   <h1 class="question_title">
    17. Letter Combinations of a Phone Number
   </h1>
   <p>
    Given a string containing digits from
    <code>
     2-9
    </code>
    inclusive, return all possible letter combinations that the number could represent.
   </p>
   <p>
    A mapping of digit to letters (just like on the telephone buttons) is given below. Note that 1 does not map to any letters.
   </p>
   <p>
    <img src="http://upload.wikimedia.org/wikipedia/commons/thumb/7/73/Telephone-keypad2.svg/200px-Telephone-keypad2.svg.png">
   </p>
   <p>
    <strong>
     Example:
    </strong>
   </p>
   <pre>
<strong>Input: </strong>"23"
<strong>Output:</strong> ["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"].
</pre>
   <p>
    <strong>
     Note:
    </strong>
   </p>
   <p>
    Although the above answer is in lexicographical order, your answer could be in any order you want.
   </p>
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   <h1 class="question_title">
    17. 电话号码的字母组合
   </h1>
   <p>
    给定一个仅包含数字&nbsp;
    <code>
     2-9
    </code>
    &nbsp;的字符串，返回所有它能表示的字母组合。
   </p>
   <p>
    给出数字到字母的映射如下（与电话按键相同）。注意 1 不对应任何字母。
   </p>
   <p>
    <img src="http://upload.wikimedia.org/wikipedia/commons/thumb/7/73/Telephone-keypad2.svg/200px-Telephone-keypad2.svg.png">
   </p>
   <p>
    <strong>
     示例:
    </strong>
   </p>
   <pre><strong>输入：</strong>"23"
<strong>输出：</strong>["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"].
</pre>
   <p>
    <strong>
     说明:
    </strong>
    <br>
    尽管上面的答案是按字典序排列的，但是你可以任意选择答案输出的顺序。
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